1. Field of the Invention
The present invention relates to a data communication system. More particularly, it relates to a polling data communication system in which, first, a polling is carried out from a center office to a plurality of terminal blocks, each consisting of terminals, to detect a request for a data transfer therefrom, and second, a polling is carried out from the center office to all of the terminals in the terminal block which issues the request.
2. Description of the Related Art
In a data communication system including a center office and a plurality of terminals connected to the center office through a cable, for example, in a cable television (CATV) system, polling is employed to communicate between the center office and the terminals. More specifically, the center office sequentially sends an inquiry signal to all terminals by a polling method to detect requests for a data transfer from the center office to terminals requesting the data transfer. However, this simple terminal polling method suffers from a drawback of superfluous pollings, because not all of the terminals always request the data transfer, and thus unnecessary pollings of terminals which have not requested a data transfer is carried out. As a result, this simple terminal polling method suffers from a drawback of a long polling time.
To overcome this drawback, a block (or group) polling method is employed. The terminals are previously grouped into a plurality of blocks, each block having an equal number of terminals. First, the center office sequentially sends an inquiry signal to the terminal blocks and detects the existence of a data transfer request(s) from the terminal(s) in each terminal block. Second, the center office sequentially sends an inquiry signal to all terminals which are grouped in the terminal blocks in which the data transfer requests were made. On the other hand, the center office does not carry out a polling of terminals grouped in the terminal blocks in which no terminal has requested a data transfer. As a result, the total polling number can be reduced, thus shortening the total polling time.
Generally, the frequency of data transfer requests varies in accordance with the time, the day of the week, regions where the terminals are installed, programs provided from the center office, etc. For example, in the CATV system, demands from a region wherein most people work during the day are low during the daytime on weekdays, but are high at night. Also, the demands depend upon the programs provided from the center office. However, the terminals grouped in each block are fixed, and the number of the terminals in each block is constant. As a result, the prior art block polling system still suffers from a drawback of superfluous pollings during the day.
The prior art block polling data communication system will be described later with reference to the drawings.